1. Field of the Invention
The present invention relates to the correction or compensation of amplifier input offsets and the control of low frequency variations in amplifier input signals, and more particularly, control of amplifier input offsets and control of low frequency variations in amplifier input signals utilized in data detection circuits such as, for example, data detection circuits used with optical data disks.
2. Description of Related Art
In many data detection circuits an amplifier is used to boost an electrical signal received from a data storage media, such as a CD-ROM, DVD, or other optical disk, magnetic hard disk, etc. In the case of optical disks, the electrical signal is generated from light that is reflected off an optical disk and converted to electrical pulses. The electrical pulses may then be transmitted to the front end of a variable gain amplifier prior for further signal processing. Data detection circuits may also be combined with circuitry for write operations. For example, circuitry for both read and write operations may be combined read/write channel circuits utilized with magnetic hard disks. In contrast, some optical disks are utilized in read only systems and thus the data detection circuit need not be combined with write circuitry.
In many data detection applications, the received electrical signal represents a corrupted digital signal which is reconstructed downstream of the amplifier. In these applications, the spectral content of the received signal may have significant energy at very low frequencies. In addition to the low frequency information content, the input offset voltage of the amplifier is frequently as large or larger than the amplitude of the received signal making it imperative that an offset cancellation scheme be used to cancel the DC offset of the amplifier while still passing the low frequency and high frequency signal spectra.
When implementing amplifiers in the data detection circuitry of optical storage systems, additional concerns exist. For example, the optical pickup (or read head) may operate at a higher supply voltage level than the signal processing circuitry of the data detection circuit. Typical optical pickups may operate at voltage levels such as 5 volts whereas it may be desirable to implement data detection circuit using a 3.3 volt supply or less CMOS circuitry. Thus, it would be desirable to protect the amplifier circuitry from the higher voltage levels produced by the optical pickup without distorting the data signal. Furthermore, because of the very high speed signals encountered with optical disks, it is desirable that the amplifier have a wide enough bandwidth to be able to process the data without attenuating the high frequency information. To optimize the amplifier bandwidth vs. current usage relationship, it is desirable to operate the amplifier in a region where the quiescent point is approximately half the supply voltage so that the whole dynamic range of the amplifier may be utilized while maximum bias current flows in the amplifier. To achieve such results, compensation of the input offset voltage of the amplifier (which is an inherent error source in amplifiers and which may result from a number of factors) must be performed.
Attempts to maintain the voltage levels at the inputs to the variable gain amplifier relatively constant to maximize the dynamic range of the amplifier are further complicated in that the signals from the optical pickup may exhibit low frequency variations. The data rates transmitted from the data storage media may typically be centered at 25 to 30 MHz and the low frequency variations typically less than 5 kHz and more generally from 1 kHz to DC. Such input signal low frequency variations may result from a number of undesirable factors including the inaccuracies in the tracking ability of the optical pickup, thermal drift of the transimpedance amplifier within the optical pickup, disk reflectivity variations, and disk spin flutter. Thus, it is desirable to control the low frequency variations in amplifier input signals.
Optical disks also present challenges in that defects on the disk (for example, particles or disk manufacturing defects) may cause low frequency disturbances, interruptions, gaps, data spikes, data dropouts, data occlusions or other data abnormalities (collectively called data disturbance) in the input signal. These defects thus complicate the input offset compensation and control of the low frequency variations in amplifier input signals since it may be desirable to hold constant (or "freeze") offset adjustments when such defects are encountered.
In the prior art, methods for passing low frequency signals to variable gain amplifiers without passing unwanted large low frequency variations have included simple AC coupling techniques that use resistors and large value capacitors to couple signals onto an integrated circuit. Because of long time constants (low frequency cutoff), the capacitor is large. Large capacitors require prohibitively large areas and are not typically integrated with the amplifier. Thus, these capacitors are off-chip (i.e. not within the integrated circuit which contains the amplifier and other data detection circuitry) as shown in FIG. 1. FIG. 1 illustrates a signal source V.sub.sig 10 (for example a signal from an optical disk) which is amplified with a variable gain amplifier (VGA) 16. The amplifier output 18 is then provided for further signal processing by the remaining portions of the data detection circuitry which may include, for example, an analog to digital converter (ADC) 17. Dashed line 15 indicates a division between off-chip structures (V.sub.sig 10 and the capacitor C.sub.in 12) and the on-chip structures (VGA 16 and the resistor 14). Disadvantages of the above approach are that an off-chip capacitor is required, and a separate circuit is needed to compensate the input offset voltage of the variable gain amplifier. Moreover, such systems generally are unable to freeze or hold the low frequency control and offset compensation during data disturbances. Further, servo information is often present at very low frequencies and the use of off-chip capacitors would lose such information prior to the data inputs of a data detection circuit. Because many data detection systems are highly integrated and incorporate servo circuitry, such a loss of information is generally unacceptable.
Prior art continuous time input offset cancellation methods have included control loops which sense the output offset of an amplifier and then feedback a voltage or current signal that cancels the random offset built into the amplifier. These circuits can also be used to cancel offsets from previous amplifier stages. FIG. 2 shows an example of such system. FIG. 2 illustrates the use of an analog servo loop technique for canceling offsets. As shown in FIG. 2, a feedback loop from the outputs 18 of the VGA 16 is provided through an analog low pass filter (LPF) 22 and analog control circuitry 20. However, the capability of freezing the amplifier control is not possible when defects or gaps are encountered. Also, the size of on chip capacitor(s) required in the low pass filter are still prohibitive when the coupling frequency is low.